Check valves generally are well known. Many hydraulic and pneumatic systems require check valves to achieve unidirectional fluid flow associated with one polarity of differential pressure across the check valve, and zero or negligible flow when the polarity of the differential pressure is reversed. Such check valve may be used to establish a priority of direction of fluid flow where the magnitude of the differential pressure across the check valve when flowing is small with respect to system operating pressures. Such check valve may also be used to relieve undesirably high pressure conditions. In these cases, the differential pressure across the check valve when flowing corresponds to the desired level of pressure relief.
A common objective of all check valves is to have zero or negligible flow, so-called leakage flow, when the check valve is supposedly fully shut off Conventional mechanizations of check valves include, ball and seat, poppet and seat, disc and nozzle tip, and diaphragm and nozzle tip.
In the operation of conventional check valves, when the differential force associated with opening the check valve, such as created by differential pressure or by other means, exceeds a bias force, usually created by a spring, the movable valve closure unseats or opens to pass fluid flow. The opening of the check valve, within limits of its operating travel, is generally proportional to the differential force. When the differential force is removed suddenly, the check valve closes rapidly and the valve closure seats positively. This mode of operation has given acceptably low leakage across the check valve.
A problem occurs during closing when the differential force across the check valve slowly decays to zero. The moving closure element of the check valve fails to seat positively and excessive leakage often occurs. This may be caused by the presence of micronic contaminant in the fluid that collects at the check valve seat when the moving closure element is almost bottomed or seated. This same silting phenomenon may occur if the differential force barely exceeds the magnitude of the bias force such that the valve fails to open fully prior to resealing. Another cause of the problem may be the presence of friction on the moving closure element that prevents it from finally seating. There may be other physical alignment problems which cause cocking or the like to prevent firm seating.